Add Rust test suite + CI

[roboshyim]

Add unit/integration tests and a GitHub Actions workflow.

Tests:

• normalize_uri drops tracking params + sorts query
• pass-path matching
• cache key generation varying by sw-cache-hash header and sw-currency cookie
• /widgets/checkout/info short-circuit logic
• purge tag parsing

CI:

• cargo fmt (check)
• cargo clippy (deny warnings)
• cargo test

Note: keep tests runnable without external services.

[roboshyim]

Implemented via PRs #7 (tests+CI), #8 (normalization+tests), #9 (purge-by-url), #10 (ban), #11 (stale-if-error). All merged.